How can i make the railing follow the curve on these stairs?
To make a non-deformed duplications, you must not use Array and Curve but Duplication, called Instancing in 2.8. 2.8 makes it a bit more complicated than 2.7 in my opinion, but if you don't need to keep a bezier curve you can do it with Instancing > Verts:


Convert your curve to a mesh.





Give it an Instancing > Verts.





Select your pillar, delete the modifiers, parent it to the curve. It should be duplicated along the curve.





You can equalize the gap between the vertices with a W > LoopTools > Space. If you need to move a pillar, just move its corresponding vertex. Also, you can hide the original pillar if needed.